The present invention relates to a head up display system and a method of overlaying a generated image on a scene.
A typical prior art head up display comprises an image generator arranged to generate an image at an object surface thereof, a combiner arranged to overlay the generated image on a scene viewed through the combiner and an optical system disposed between the image generator and the combiner to convey the generated image from the image generator to the combiner. It will be understood that the image formed by the image generator follows an optical path from the object surface, through the optical system, to the combiner.
When such a head up display is used as a pilot""s display system in an aircraft cockpit, to accommodate the display within the spatial constraints of the aircraft cockpit, the optical path between the object surface and the combiner is usually folded within the optical system in such a manner that the generated image is redirected between the image generator and the combiner. In such a system, the optical system may comprise a reflector positioned below the combiner, in line with the pilot""s ejection plane of the cockpit, such that there is no room available forward of the ejection plane to accommodate additional display arrangements such as a head down display system.
In International Patent Application Publication WO 89/12840 there is described a display which uses two spaced reflective surfaces to reflect an image from three positions arranged along the two reflective surfaces to transfer the image to the combiner. That is the image is generated at an object surface and is redirected upwardly onto the combiner by a lower fully reflective surface and an upper reflective surface having both light transmissive and light reflective properties provided by a semi-silver coating. The two reflective surfaces are arranged to face one another and to taper towards one another in the direction of propagation off the image through the system. In operation, the image from the object surface is reflected first at the lower reflective surface and then reflected, in part, at the upper reflective surface back to the lower surface where it is again reflected to the upper reflective surface for transmission therethrough to the combiner for reflection to an observer.
In this manner, a more compact system is constructed which has a shallower profile than obtained using a reflector which provides a single reflection. However, the transmission efficiency of this system is poor and susceptible to unwanted reflections from various surfaces within the aircraft cockpit and the system.
It is an object of the present invention to obviate or mitigate the disadvantages associated with the prior art.
According to a first aspect of the present invention a head up display system comprises an image generator arranged to generate an image, a combiner, and an optical sub-system disposed between the image generator and the combiner having a prism comprising first and second reflective surfaces and a mirror having a mirror surface, the prism and the mirror being arranged to convey a generated image from the image generator to the combiner, wherein the prism is arranged to internally reflect a generated image from a first portion of the first reflective surface towards a first portion of the second reflective surface arranged to internally reflect the image through a second portion of the first reflective surface arranged to allow the image to pass therethrough for reflection by the mirror surface of the mirror that is arranged to reflect the image back through the second portion of the first reflective surface and through a second portion of the second reflective surface, arranged to allow the image to pass therethrough, to a reflective surface of the combiner, arranged to overlay the image on a scene viewed through the combiner.
Preferably, the image follows an optical path way through the system and the prism may taper inwardly in the direction of propagation of the image along the optical path way. The first and second reflective surfaces may be planar.
The first portion of the second reflective surface may have a coating on a surface of the prism that is otherwise an optically transparent material. The coating may be a dielectric coating.
The optical sub-system may also comprise a relay lens arrangement disposed between the image generator and the combiner which is arranged to convey an image generated by the image generator to the first portion of the first reflective surface of the optical sub-system.
Preferably, the combiner may be a holographic combiner. The mirror surface of the mirror may be substantially flat. Alternatively, the mirror surface of the mirror may be spherical.
A polarising filter may be disposed between the prism and the combiner.
The combiner may be positioned above the optical sub-system when the system is mounted in an aircraft cockpit for use as a head up display system for a pilot.
According to a second aspect of the invention a method of overlaying a generated image on a scene, includes generating an image, internally reflecting the image from a first portion of a first reflective surface towards a first portion of a second reflective surface, internally reflecting the image through a second portion of the first reflective surface arranged to allow the image to pass therethrough, reflecting the image from a mirror surface back through the second portion of the first reflective surface and through a second portion of the second reflective surface arranged to allow the image to pass therethrough to a reflective surface of a combiner, and overlaying the image on a scene viewed through the combiner.